1. Field of the Invention
This invention relates to cell-type devices characterized by a pair of electrodes separated by an electrolyte and particularly to thermal galvanic cells. More specifically, this invention is directed to the conversion of thermal energy into electrical energy. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly applicable to the conversion of thermal energy into electrical energy and especially to thermal galvanic cells for effecting such energy conversion. The present state of the art with regard to thermal galvanic cells is believed to be exemplified by the disclosure of U.S. Pat. No. 4,211,828. The disclosure of the said U.S. Pat. No. 4,211,828 is hereby incorporated herein by reference.
The cells of the referenced patent represent a significant step forward in the art of thermoelectric energy systems. These improved cells are characterized by a pair of electrodes which are separated by an electrolyte. The generation of current by the cell is induced through the establishment of a temperature gradient across the cell and specifically by causing the two electrodes to assume different temperatures.
There is, of course, an ever present desire to enhance the efficiency of all devices and this is particularly true in the case of energy conversion and especially those devices or systems wherein electrical energy is directly produced from thermal energy. In the case of the cells of the referenced patent, enhanced performance may be measured in terms of electrode efficiency. Electrode efficiency, as this term is employed herein, is the electrode/electrolyte interfacial resistivity. Cell efficiency is defined as the electrical output power divided by the thermal power input. Cell performance may be measured in terms of the cell resistance per unit area of electrode (ohm cm.sup.2). The latter quantity, while not resistivity in the classical sense; i.e., not ohm cm; will be referred to as such herein and will mean the externally measured resistance of the cell multiplied by the area of one of the electrodes. The open circuit voltage, V, of a thermal galvanic cell of the type being discussed is a function of the temperature difference between a pair of oppositely polarized electrodes. The open circuit voltage may be obtained by multiplying temperature difference by the "Seebeck" coefficient. The "Seebeck" coefficient is also known as the thermal galvanic cell constant of thermoelectric power, in common usage, is defined as the change in open circuit voltage per degree Celsius expressed as mv/.degree.C. The output power which may be derived from a thermal galvanic cell may be expressed as: EQU P=I.sup.2 R (1)
where:
R is the load resistance; and PA1 I may be expressed as: ##EQU1## where: V is the open circuit voltage; and PA1 Ri is the internal cell resistance as per Thevenin's theorem. For maximum power output, as can be proven by maximizing the derivative of P with respect to R, Ri must be equal to R. Combining equations (1) and (2) above: EQU P=V.sup.2 /4Ri (3) PA1 S is the Seebeck coefficient; and PA1 dT.sub.1 is the temperature difference between the two electrodes.
The open circuit voltage for a thermal galvanic cell is: EQU V=(S) (dT.sub.1) (4)
where:
Therefore, equation (3) above reduces to: EQU P=(SdT).sup.2 /4Ri (5)
Accordingly, for any given cell with a constant Seebeck coefficient, the power output for cells of equal geometries and temperatures is a function of internal resistance only. If the power per unit area is desired, for purposes of comparison, the above definition of resistivity may be employed since, under equal temperature differentials, the voltages would be the same. In summation, the efficiency of the thermal galvanic cell of the referenced patent could be enhanced if the resistance per unit area of electrode of the cell, hereinafter the "resistivity", could be minimized.